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Insane in the Membrane: Modification of Polymeric Membranes for High Performance Water Separations

  • Author(s): McVerry, Brian
  • Advisor(s): Kaner, Richard
  • et al.
Abstract

It is not unknown that the worldwide freshwater supply is dwindling and current freshwater sources are being contaminated. The total amount of freshwater, which makes up 2.5- 3% of the total water on earth, is actually sufficient to sustain the growing population for many years. However, the disproportionate geographical distribution of freshwater raises issues, as many people occupy arid regions. The transport or piping of large volumes of water over long distances is typically prohibitively expensive and/or prevented by geopolitical interests. The drive to produce freshwater locally from non-traditional sources has advanced in recent years and current energy and production costs have fallen due to technological advances.

Filtration through polymeric materials has emerged as the leading technology for freshwater production from seawater, brackish water, and wastewater. Polymer membranes have surpassed ceramic membranes as the leading filtration medium because of their low-cost and

versatility. Large sheets of membranes can be cast using an in-line manufacturing process and pore sizes can be readily tuned for specific applications. However, membrane fouling severely limits the advantages and economics of polymeric membrane-based separations. This dissertation will examine methods to produce fouling-resistant membranes that maintain the low- cost and scalability of polymer membranes for ultrafiltration and reverse osmosis.

The first chapter presents a brief history and introduction to membrane technology and its current status. This chapter outlines membrane casting methods, the polymers used in casting films and their advantages, different configurations for membrane modules, the effects of fouling, and the cost of fouling.

The second chapter introduces a new hydrophilic, polyaniline-based additive that was synthesized to blend into polysulfone unltrafiltration membranes. Sulfonated polyaniline (SPANi) is a self-doped polyaniline derivative that is readily synthesized using fuming sulfuric acid. SPANi, when de-doped, is soluble in polar organic solvents and insoluble in water, making it an excellent candidate for blending into ultrafiltration membranes. When redoped at pH 7, the presence of pendant sulfonic acid groups creates a zwitterionic species. Zwitterionic moeities are known to possess super-hydrophilic and ultra-low fouling properties. When blended into polysulfone (PSf) ultrafiltration membranes at low concentrations, the SPANi-PSf membranes demonstrate greater hydrophilicity and anti-fouling properties when compared to pure PSf membranes with nominal changes in performance.

For reverse osmosis (RO) membranes, the active or “skin” layer that performs the separation is a dense, non-porous polymer. Thus, blending hydrophilic molecules into the active layer or modifying the chemistry of the active layer affects the separation properties of the resulting polymer blend. To address this issue, researchers have grafted hydrophilic polymers to

the surface of membranes to impart anti-fouling properties without significant changes in their performance. In Chapter 3, a new method is presented using photoactive hydrophilic polymer precursors to modify membrane surfaces. With this new method, commercial RO membranes can be modified rapidly under ambient conditions, maintaining the roll-to-roll scalability of RO membrane processing. The grafted commercial RO membranes exhibit enhanced hydrophilicity and anti-fouling properties when compared with unmodified RO membranes.

Chapter 4 investigates applying the same method of imparting anti-fouling properties to polymeric membranes as discussed in Chapter 3, except here this method is applied to ultrafiltration (UF) membranes used for membrane bioreactors. Membrane bioreactor technology has gained attention recently for its highly efficient ability to clean wastewater. However, the concentrated loading of organics and microorganisms into the reactor medium leads to high fouling rates. First, modeling data are presented that identifies specific acid-base interactions between different foulants and the membrane surface that accelerate membrane fouling. New photoactive small molecule precursors were then synthesized and covalently bound to the membrane surface to repel these interactions. The small molecule with the greatest repulsive interaction, a zwitterionic derivative, imparted the greatest anti-fouling properties to the commercial UF membranes. The modified membranes exhibited low fouling rates in both a short-term and a long-term study.

Chapter 5 summarizes the dissertation and outlines the key takeaways.

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